The present invention relates to a master cylinder with reduced free travel. More particularly, the invention relates to a master cylinder piston and to a master cylinder comprising a piston according to the invention.
Current braking systems usually comprise a master cylinder. Such master cylinders comprise a master cylinder body. A pressure chamber is made in this master cylinder body. The pressure chamber forms a bore inside the body of the master cylinder. A piston is mounted slideably in the bore formed by the pressure chamber. The piston and the bore are of matching shape. Typically, the piston and the bore have a cylindrical shape. The piston then has a diameter that is smaller and close to the diameter of the bore.
The pressure chamber is supplied with brake liquid, or more usually with hydraulic fluid, by a hydraulic fluid reservoir. The pressure chamber is connected to the hydraulic fluid reservoir via a hydraulic fluid supply duct. The supply duct is made in the body of the master cylinder. An outlet of the supply duct is facing the piston. An orifice is made in the piston in order to allow the hydraulic fluid to travel from the supply duct into the pressure chamber.
A master cylinder supplies a hydraulic circuit with hydraulic fluid. For this, the pressure chamber comprises a hydraulic fluid outlet. Said outlet is made in a bottom of the pressure chamber. This outlet is directly connected to the hydraulic circuit. To supply the circuit with hydraulic fluid, the pressure in the pressure chamber must increase. The increase in pressure in the pressure chamber discharges the hydraulic fluid contained in said pressure chamber through supply outlets of the hydraulic circuit. The fluid expelled from the pressure chamber by the increase in pressure is injected into the hydraulic circuit.
During a braking action, the piston is displaced in the bore of the master cylinder body. This displacement reduces the size, and therefore the capacity for storing hydraulic fluid, of the pressure chamber. Moreover, this displacement closes the hydraulic fluid supply of the pressure chamber. Specifically, by displacing, the orifice made in the piston is facing an inner wall of the bore and the supply duct is facing an outer wall of the piston. However, to increase the pressure of the hydraulic fluid in the pressure chamber, the closure of the hydraulic fluid supply of the pressure chamber must be sealed.
In order to ensure the seal, and therefore the increase in pressure in the pressure chamber, a groove is made in the bore of the master cylinder body. This groove is situated such that, in the working position, that is to say during a displacement of the piston in the bore, the orifice made in the piston is situated either facing said groove or more distant from the supply duct than said groove.
A sealing cup is placed in the groove. This cup plays the role of a seal. The function of such a cup is to ensure the sealing of the closure of communication between the pressure chamber and the supply duct. For this, the cup is only partially housed in the groove. More particularly, a portion of the cup comes out of the groove in order to come into contact with the piston. This contact must ensure the seal between the supply duct and the orifice of the piston when the piston is in the working position. In the working position, the portion of the seal that comes out of the groove is situated either facing the orifice of the piston, so as to block said orifice, or in contact with an outer wall of the piston closer to the supply duct than said orifice of the piston. This positioning of the cup blocks the circulation of the hydraulic fluid between the orifice of the piston and the supply duct in a sealed manner.
The cup usually has the shape of a crown having a U shaped profile. The U shaped profile comprises a base and two lips. One lip, called the inner lip, is bonded against the outer wall of the piston. One lip, called the outer lip, is bonded against a bottom of the groove. The contact between the piston and the inner lip is sealed. The contact between the bottom of the groove and the outer lip is sealed.
When the piston slides, along an axis of displacement of the piston, until the inner lip is facing the orifice of the piston or the inner lip is closer to the supply duct than the orifice of the piston, the cup obstructs in a sealed manner the supply of fluid of the pressure chamber. This obstruction allows, if the piston continues to advance in the master cylinder, a rise in pressure in the pressure cylinder. The rise in pressure makes it possible to supply the hydraulic circuit with hydraulic fluid. This supply is produced by injecting the hydraulic fluid contained in the chamber into the hydraulic circuit. The supply of hydraulic fluid of a hydraulic braking circuit allows the braking action.
During a braking action, the orifice of the piston travels a certain distance, also called the free travel, before facing the inner lip or being more distant from the supply duct than the inner lip. The lack of communication between the supply duct and the pressure chamber is not sealed during the free travel. The absence of a seal during this free travel does not allow the pressure to rise in the pressure chamber. The absence of a rise in pressure in the pressure chamber does not allow braking. During the free travel, a user of a braking device presses on a brake pedal without there being any effective braking.
In order to limit or remove the free travel, it is possible to place the orifice as near as possible to the inner lip of the cup. The orifice is then not facing the supply duct. However, if the orifice of the piston is not facing the supply duct in the rest position, the flow rate of hydraulic fluid supplying the pressure chamber may be insufficient. Specifically, the liquid present in the pressure chamber and sent into the hydraulic circuit during a braking action is not replaced rapidly if the orifice of the piston is not facing the supply duct. It is possible to advance the orifice so that said orifice is only partially facing the supply duct. This partial facing position is such that it ensures a sufficient flow rate to fill the pressure chamber rapidly while reducing the free travel. However, such a partial facing position maintains a considerable free travel.
Master cylinder devices are known that make it possible to reduce the free travel and allow the braking action to be more responsive with respect to the user pressing on the pedal. Therefore, in U.S. Pat. No. 6,272,858, braking devices are known that comprise a piston having a frustoconical portion. The frustoconical portion then forms an inclined wall relative to an axis of displacement of the piston. The orifice of the piston is then situated on said inclined wall. The inclined wall is such that the external diameter of the piston, that is to say the diameter of the piston outside the frustoconical portion, is greater than or equal to the diameter of the piston at the frustoconical portion. There is then a gap between an inner wall of the bore and the piston at this frustoconical portion.
The orifice of the piston then no longer needs to be facing the supply duct. Specifically, the orifice of the piston being situated on the inclined wall, if the supply duct is even partially facing the frustoconical portion, the gap between an inner wall of the bore delimiting the supply duct and the inclined wall allows the passage of the hydraulic fluid. Depending on the inclination of the inclined wall relative to the axis of displacement of the piston and depending on the gap between the inclined wall and the inner wall of the bore delimiting the supply duct, a sufficient flow rate to effectively fill the pressure chamber may be supplied despite the orifice of the piston and the supply duct not facing one another.
However, such a frustoconical portion, although it makes it possible to greatly reduce the free travel, creates a junction zone forming a break in the diameter of the piston. This break may prevent a fully sealed contact between the inner lip and the piston. When the inner lip is pressed on the junction zone, a first portion of the inner lip is pressed on the inclined wall, but the rigidity of said inner lip prevents a second portion of the inner lip from being in contact with the external diameter of the piston. The inner lip is then only partially in contact with the piston. The partial contact of the inner lip with the piston creates a weakness of seal that can prevent the inner lip from correctly fulfilling its role as a seal. If the seal is no longer ensured, the pressure in the pressure chamber can no longer increase correctly. A lack of supply of the hydraulic circuit and therefore a failure of braking may take place.
Such a solution therefore does not make it possible to reduce the free travel considerably while maintaining a level of seal and of flow rate sufficient to ensure the correct operation of the master cylinder. This solution is therefore not acceptable as it is.
The device according to the invention proposes to reduce the free travel considerably while ensuring a perfect seal. For this, the device according to the invention proposes a master cylinder piston comprising an inclined wall. The inclined wall according to the invention forms a convex rounded surface. The orifice of the piston is situated wholly on said convex rounded surface. The presence of the inclined wall makes it possible to ensure a sufficient flow rate to fill the pressure chamber even though the orifice of the piston is not situated facing the supply duct. The inclined wall being rounded and convex, the inner lip remains, at every point of said inner lip facing the piston, in contact with the piston during a braking action. Such a contact ensures a seal capable of guaranteeing an effective rise in pressure in the pressure chamber. A separation of a portion of the lip is prevented by the convex curvature of the inclined wall.
The subject of the invention is therefore a brake master cylinder comprising, from a brake pedal to a bottom of the master cylinder, a master cylinder body, said master cylinder body comprising a chamber delimiting a bore of the body of the master cylinder, a piston sliding in the bore on an axis, a reservoir of hydraulic liquid supplying the chamber via an orifice in the piston and a supply duct in the master cylinder body, a sealing cup, the sealing cup being in contact via an inner lip with the periphery of the piston, and via an outer lip with the body of the master cylinder, said orifice being either facing said inner lip or more distant, depending on the position in displacement of the piston on an axis of displacement of said piston, from the supply duct than the inner lip during a braking action, the piston comprises a frustoconical portion, the frustoconical portion of the piston is formed by an inclined wall relative to an axis of displacement of the piston, characterized in that said inclined wall forms an incurved convex rounded surface from an external diameter of the piston to a diameter smaller than the external diameter of the piston, said rounded surface being such that an entrance of the orifice is wholly situated in the rounded surface.
A preferred embodiment of the invention provides that the rounded surface has a progressive curvature.
A preferred embodiment of the invention provides that, in the rest position, the orifice of the piston is at most partially facing the supply duct of the body of the master cylinder.
A preferred embodiment of the invention provides that the inclined wall is facing a portion of the master cylinder body, said portion forming a wall of the supply duct.
A preferred embodiment of the invention provides that a gap is present between the portion of the body of the master cylinder facing the inclined wall in the rest position.
A preferred embodiment of the invention provides that, in any working position, the inner lip is substantially parallel to the inclined wall.
A preferred embodiment of the invention provides that, when the master cylinder is at work, the inner lip is pressed over its whole surface against the inclined wall.